This invention relates to tuned filters for correction of harmonics in electric power systems, and also relates to components for use therein.
In the power industry, filters are known which incorporate variable inductors wherein the inductor is connected as two parts in series, one of which is mechanically moveable relative to the other. It is also known to provide an iron core having a separate DC winding around the core carrying an adjustable DC control current which varies the effective reluctance of the iron core by partial cross-flux saturation, and therefore varies the inductance.
Such prior art variable inductors have difficulties respectively of mechanical unreliability, and of difficulty in providing sufficient electrical clearance to the iron core.
A particular application where variable inductors are desirable is filtering of harmonics in power systems. Commonly, in such systems a filter including an inductor is connected in shunt to the power system. The filter typically contains one or more tuned filter arms, each consisting of an inductor and capacitor in series tuned to a particular harmonic frequency.
Normally the tuned frequency of such a filter arm will be arranged to be substantially exact when the ac power system frequency is its nominal value, for example 50 Hz or 60 Hz. In this condition the total impedance of the filter arm at the relevant harmonic frequency will be small and the harmonic voltage on the busbar (where the filter is connected) at the tuned frequency will also be small, with almost all of the relevant component of the interfering harmonic current diverted into the filter arm.
However, it will be appreciated that the frequency of an AC power system is rarely exact, and in some cases variations of several percent may occur. In this case, the filter will detune, presenting a relatively high reactance, so that its filtering effect is degraded and unacceptable ac harmonic voltage may occur on the busbar. A similar effect can also occur if the initial adjustment of component values in the filter is inaccurate, or if the component values subsequently change, for example due to change of ambient temperature.
It will therefore be appreciated that variable inductors having an enhanced and accurately achievable variability within the filter may be beneficial in order to help compensate for these variations.
According to a first aspect of the invention there is provided a tuned filter for connection to an electric power system, the filter incorporating a variable inductor assembly, the variable inductor assembly comprising;
a main winding,
a plurality of auxiliary windings arranged to act in inductive series with respect to the main winding, and
switching means associated with each auxiliary winding for selectively connecting the auxiliary winding in electrical series with the main winding, each auxiliary winding having a number of turns substantially equal to x2(nxe2x88x921) where n is a positive integer corresponding to the position of each auxiliary winding in the inductive series of auxiliary windings and x is also a positive integer.
Varying the value of the inductor allows the characteristics of the filter to be changed and thus can allow the frequencies filtered by the filter to be varied. It would also be possible to account for variations (for example due to temperature change) in the values of components by adjusting the inductor value.
The sequence x2(nxe2x88x921) is a binary sequence. For example, a first auxiliary winding may have a single turn, a second auxiliary winding may have two turns, a third auxiliary winding may have four turns, and a fourth auxiliary winding may have eight turns. Such a sequence is advantageous because, in conjunction with the ability to switch the auxiliary windings in and out of series with the main winding, it enables enhanced variability of the inductance.
Preferably, the switching means are constituted such that each auxiliary winding is selectively connectable to the main winding in any one of three connection modes, comprising a first connection mode in which the auxiliary winding assists the inductance of the main winding, a second connection mode in which the auxiliary winding opposes the inductance of the main winding, and a third connection mode in which the auxiliary winding is bypassed with respect to the main winding.
Such an arrangement provides a particularly efficient use of the auxiliary and main windings because it provides three possible ways for the inductance of the inductor to be influenced by the or each auxiliary winding, that is, in opposing mode, assisting mode, and no influence (bypassed) mode. It allows the inductance of the inductor to be varied upwards or downwards in a large number of steps between having all of the auxiliary windings assisting the main inductance and all of the windings opposing the main inductance.
The number of auxiliary windings in the filter will be dictated by the characteristics of the power system and the degree of variability of the inductance necessary to compensate accurately for the associated harmonics, but three or four may give sufficient variability for many purposes.
Each switching means may comprise a bridge circuit having four switching arrangements therein, the auxiliary winding associated with the switching means being connected across the bridge circuit. Preferably the bridge circuit comprises two parallel paths, each path having two series connected switching arrangements therein, the auxiliary winding being connected across the bridge circuit at a point in each path located between the switching arrangements in said path, i.e., an end of the auxiliary winding is connected between each switching arrangement of each path. Such a structure is convenient for providing the functionality described before.
Each switching arrangement may comprise first and second parallel current paths, each path comprising at least one current switching component, such as a mechanical or electro-mechanical switch. However, in the preferred embodiments the switching components comprise semiconductor devices. For example, each current path may comprise a thyristor, the thyristors being connected in reverse parallel so that current of different polarities passes through the parallel paths in opposite directions, i.e., AC current can pass through the switching arrangement. In yet another embodiment each current path may comprise two diodes in series, the diodes in the first path having their anodes connected together and the diodes in the second path having their cathodes connected together, a thyristor being connected between the anodes of the diodes in the first path and the cathodes of the diodes in the second path such that AC current can be passed through the switching arrangement when the thyristor is turned on.
The skilled person will appreciate that the working voltage across each auxiliary winding and therefore across the switching means and switching arrangements associated with each auxiliary winding depends principally on the total current flow in the filter and on the number of turns in the particular auxiliary winding. Therefore, for auxiliary windings having numbers of turns which are later in the chosen binary sequence, it may be desirable to reduce the number of turns below that dictated by the binary sequence because the working voltage across the auxiliary winding may become inconveniently large. Nevertheless, a practical arrangement of auxiliary windings will have binary number multiples in at least the lower part of the series, e.g., 3, 6, 12, 24, 24 . . . .
A control means may be provided, adapted to control the switching means. Preferably, the control means is provided within a closed loop. The closed loop is preferably adapted to vary the value of the inductor to maintain a pre-determined characteristic of the filter. The pre-determined characteristic may be a defining frequency of the filter, normally that giving minimum filter impedance at the desired frequency of filtering. The predetermined characteristic may be to minimize the amplitude of harmonics on a power supply to which the filter is connected.
The auxiliary windings are substantially co-axial with the main windings. Further, the auxiliary windings may be located adjacent an end region of the main windings. Alternatively, or additionally, the auxiliary windings may surround the main winding (that is, they are provided outside the main winding) or the main winding may surround the auxiliary winding, the latter arrangement providing what is likely to be a more compact inductor. In either case, the auxiliary winding may be provided at an end region of the main winding or may be at a central region of the main winding. An advantage of winding the auxiliary windings around the main winding or vice-versa, is that the mutual inductance between the windings is increased (when compared to placing the auxiliary winding adjacent the main winding) which reduces the voltages across the switching means and switching arrangements.
The filter may be adapted to filter multiphase power supplies, in particular three phase power supplies. In such cases, a tuned filter arm may be provided for each phase. Alternatively the filter may be adapted to filter a DC power supply provided by an AC/DC converter or other source.
It may be desirable to earth an end of the filter. In such arrangements an end of the auxiliary winding opposite to the end connected to the main winding may be earthed. This has the advantage that voltages to earth from any point of the auxiliary windings are relatively small, which makes the auxiliary windings easier to mount.
The filter may be adapted to filter either AC or DC power systems. The voltage of the power supply may be up to hundreds of thousands of volts.
The filter may comprise a capacitor in series with the inductor. Alternatively, or additionally, other components (for example a capacitor) may be connected in parallel with the inductor.
The filter may comprise parallel current paths, each path having one or more of the following components therein: capacitive, inductive, resistive. In preferred embodiments a variable inductor as previously described is provided in each current path. Such an arrangement allows the filter to be tuned to two different frequencies and for the inductors to be varied independently allowing compensation for changes in the frequencies to which the filters are tuned. Thus, the performance of the filters may be more satisfactory.
Normally, one capacitor is associated with each inductor, but in some circumstances a capacitor may be provided in association with more than one inductor. The skilled person will appreciate that an inductor/capacitor combination provides a filter, but that it may be convenient to provide the capacitance for each inductor in different ways. An advantage of providing a capacitor in association with more than one inductor is that it can reduce the number of components connected to a high voltage supply. Components for a high voltage supply are expensive and it therefore may be desirable to reduce the number connected.
The invention also provides a power distribution system having a filter with switched variable inductance as previously described fitted thereto.